What is max weight you expect the tripod to hold (camera lens flash ) then add some for safety.
Do I intend to carry it about all day or is it to be mainly static.
What price range am I willing to pay.
What kind of head do I want to use on the tripod
Do I want a quick release of the camera on the tripod.
Would I have to add a weight to hold the tripod down, ie camera bag

If you can give some idea then it would help. Check out my Manfrotto in my kit, solid and won't move in a wind but heavy. the Velbon is light weight

If trying them out in a shop take camera and heavy lens, extend the tripod to its maximum height and take a photo, see how much movement the tripod gives with shutter click. A good quick way of testing stability, nothing worse than having to do this in the field only to find a blurred photo. Would suggest a remote shutter release to go with it.

Budget - about £150, really - though could possibly stretch it a bit. Weight, to be safe about 6/7 kg? is that normal? It will mostly be static, but I don't mind carrying a fair bit of weight. Head wise, I don't really know, something that' good for bird photography, especially panning, as I do *Want* to do flight shots, but there's little opportunity to practice here atm. I don't know whether or not I need quick release - I don't think so especially, though it may be useful I guess.

Ok Panning, don't use a tripod to catch a bird in flight nigh on impossible, far easier hand held.
weight put camera+ heaviest lens and flash on kitchen scales then add a bit

I went for Manfrotto because ever camera shop should stock them and they have a good range of add- on gear but there are others just as good.

Quick release I would strongy recommend, it is suprising how often you would want to go from a static shot ( landscape- building- studio) to a moving shot where handheld is best.

Buying a tripod is like buying a camera, different ones suit different people, so you need to get one that your happy with, don't go for anything too cheap because thats what you will get, cheap rubbish that may well fall apart after a short time.

As to heads there are ones with arms and those without, I found those with arms tend to get in the way which is why I went for the Manfrotto 322 RC2 head. Its a pistol grip with tensioner. It is quick to get the camera set up to what you want to take and locks really solid when you have framed the picture. I personally find it a brilliant bit of kit and accommodates quick release as well.

Would strongly recommend you check it out.

Just one other point, it is not always the case the tripod is wanted upright, sometimes you may need to take something at ground level and the Manfrotto of mine goes right down without having to remove the centre column, there is a clever release system which allowes the centre column to go horizonal without removing it completely from the frame.